1

I need a bench for our kitchen table, and am trying to teach myself basic joinery, so I'm attempting to build one from some scrap pallet wood.

The wood is nominally 1x4 and I think is probably pine, though that's a lesser concern. As I'm new to this, I'd like feedback to ensure the design itself isn't critically flawed.

The expected load will be various combinations of my three kids (ages 9, 6, and 2). They all fidget, though the only one I expect will jump on it would be the 2 year old.

Here's the basic design: Screenshot from Sketchup

The top connects to the legs by a series of through mortises holding the outer planks in place. The inner plank is held by a dado that notches into a bit of wood acts as a spacer (I'm not sure if this is properly a tenon or not). This serves two functions: it widens the seat slightly, and minimizes the chance that spills will accumulate food gunk between the planks of the bench top.

Here's a closeup of the connection between the legs and top: Connection between legs and top, over and under views

Here's a shot with the top planks hidden, showing the location and size of the tenons: Tenons at top of legs

The thinnest place the wood gets is at the half-laps which connect the legs together. I've tried to mitigate this by compressing the half-lap between the tusk and the shoulder of the tenon, though I don't know how effective this will be: Detail of half-lap compression

In the short term, it looks like it'll be ok - I'm currently adding the lower struts as there was some racking when we tried it out without them and my kids started to fidget.

I'm a bit concerned on how long it's likely to last, and if it's probably going to break, any tips on making the next version more durable would be very welcome.

|improve this question
  • 1
    Overall I think it looks fine. The thing I'd probably worry about most is the thickness of the legs in the half-lap joints, but short of a total redesign I don't know what you could do about that. – SaSSafraS1232 Sep 20 '18 at 16:38
  • 2
    One tip for future projects that I wanted to add in case the SketchUp images exactly reflect the way this is built, use shallower angles for your wedges. 8-10° is good, these look like they're closer to 30° which is way too steep unless the tusk tenons are purely decorative. – Graphus Sep 21 '18 at 13:54
  • 1
    Thanks @Graphus, that's exactly the sort of feedback I need. Mainly I'm looking for design feedback. If the wood's not able to handle the stresses that's one thing, I'd like to avoid reworking the design if that's not the problem. I'll edit the question to be more clear about that – Morgen Sep 21 '18 at 18:22
  • 1
    Some further feedback on this one then, I think it's very likely that you should glue the overlap joints (the half-laps) and if necessary add a couple of screw too. The tusk tenon through those would be intended to hold each pair of legs quite firmly together, but that is predicated on everything being cut good and tight and the wedge finally tensioning everything nicely. If those overlaps are able to shift even slightly the whole assembly can have bit of play, so rigidly fixed you'll gain a lot of stability. I think you'll be surprised by how much difference it makes. – Graphus Sep 22 '18 at 15:56
  • 2
    Given that the mortise/tenon is already done, I'd leave everything as is and file the alternate details away for the next iteration. (If the current one turned out really wobbly, a subtle L bracket underneath would probably help with the racking.) – Aloysius Defenestrate Sep 23 '18 at 18:32
1

If the occupants of the bench rock to the left and right, the wedge will apply a large force on the end of the pegged tenon. The expected failure point is marked in the image. A combined weight of 150lbs, doing moderate rocking, could cause this failure in a few seconds.

expected failure point in pegged tenon

|improve this answer
  • Good to know, are there ways to mitigate this? I'd imagine this would be a weakness in most tables and benches that use wedged tenons in their uprights. – Morgen Sep 23 '18 at 1:41
  • 1
    @Morgen, this is a possible failure but the risk is being overstated considerably, because whether it can occur at all is entirely dependent on the flow of grain through the tenon, which isn't known. And even if the grain is in the worst possible orientation (not at all likely) it's still not a guaranteed failure. But since you asked there are two main ways to reduce the risk of this, #1 is off the table since your bench is already made so it's option #2, which is to reinforce the end of the tenon with a tight-fitting dowel glued in horizontally. Hardwood dowel ideally. – Graphus Sep 23 '18 at 13:10
  • 1
    The tusk tenon will be substantially stronger if a long mortise is made on the thin side. This would leave more wood to hold the end of the horizontal support together. Refer to this video on wedged tusk tenons: pbs.org/video/woodwrights-shop-wedged-tusk-tenon – Stephen Meschke Sep 23 '18 at 14:31
  • 1
    @StephenMeschke thanks for the link, it was really informative. I'm going to recut the keys at a gentler angle, and adjust the design for a longer tenon (for the next one). As is, I think it'll be OK, as the tenon ended up being a bit larger than in the design (I got lazy about trimming them down) – Morgen Sep 23 '18 at 21:50
1

The weakest point ended up being the half-laps (as predicted by @SaSSafraS1232):

Lengthwise crack in the leg

|improve this answer
  • SE is not like a forum, it's a Q&A site and you've posted where an Answer should go. You should delete this and add it as an update to the Question. – Graphus Oct 18 '18 at 13:14
  • True, SE is not a forum, however self-answered questions are permissible. In this case, I elected to put this as an answer for two reasons: 1. It is an answer to the question, and answering a question in the question itself doesn't seem appropriate. 2. The correct answer had been left as a comment, so my intention was also to convert that information into an actual answer. Should @SaSSafraS1232 be willing to swing back and convert their comment into an answer, I'd be inclined to upvote that and delete this. – Morgen Oct 18 '18 at 14:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.